Chlamydomonas reinhardtii

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Chlamydomonas reinhardtii is a motile single celled green alga about 10 micrometres in diameter that swims with two flagella. See Chlamydomonas.

These algae are commonly found in soil and fresh water. They have a cell wall, a large cup-shaped chloroplast and an "eye" (the pyrenoid) that senses light. Normal Chlamydomonas can grow on a simple medium of inorganic salts in the light, using photosynthesis to provide energy. They can also grow in total darkness if acetate is provided as a carbon source.

Vegetative cells of the reinhardtii species are haploid with 19 small chromosomes. Under nitrogen starvation, haploid gametes develop. There are two mating types, identical in appearance and known as mt(+) and mt(-), which can fuse to form a diploid zygote. The zygote is not flagellated, and it serves as a dormant form of the species in the soil. In the light the zygote undergoes meiosis and releases four flagellated haploid cells that resume the vegetative life cycle.

The origin of the C. reinhardtii WT laboratory strain c137 (mt+) is uncertain, but it is thought to have been collected from a field in New England in the 1940s.

Curious fact: Under ideal growth conditions, cells may undergo two or three rounds of mitosis before the daughter cells are released from the old cell wall into the medium. Thus, a single growth step may result in 4 or 8 daughter cells per mother cell.

Chlamydomonas is used as a model organism for research on fundamental questions in cell and molecular biology such as:

How do cells move?
How do cells respond to light?
How do cells recognize one another?
How do cells regulate their proteome to control flagellar length?
How do cells repond to changes in mineral nutrition? (nitrogen, sulfur etc.)

There are many known mutants of C. reinhardtii. These mutants are useful tools for studying a variety of biological processes, including flagellar motility, photosynthesis, protein synthesis, etc. The attractiveness of the alga as a model organism has recently increased with the release of several genomic resources to the public domain. A draft of the Chlamydomonas genome sequence was completed in February 2003 by the Joint Genome Institute of the U.S. Dept of Energy. The sequences of all three C. reinhardtii genomes are available. The ~15.8 Kb mitochondrial genome (database accession: NC_ 001638) is available online at the NCBI database (http://www.ncbi.nlm.nih.gov/). The complete >200 Kb chloroplast genome is available online (http://www.chlamy.org/chloro.html). The 2003 second draft of the alga’s ~125 Mb nuclear genome is also currently available online (http://genome.jgi-psf.org/Chlre3/Chlre3.home.html) in which the 1.8 million reads have been assembled into 3211 scaffolds using the whole-genome shotgun approach. In addition to genomic sequence data there is a large supply of expression sequence data available as cDNA libraries and expressed sequence tags (ESTs). Seven cDNA libraries are available online (http://genome.jgi-psf.org/chlre2/chlre2.home.html) and for purchase from the Clemson University Genomics Institute (https://www.genome.clemson.edu/). There are also two databases of >50 000 (http://www.kazusa.or.jp/en/plant/chlamy/EST/) and >160 000 (http://www.chlamy.org/search.html) ESTs available online.

C. reinhardtii DNA transformation techniques

Gene transformation occurs mainly by homologous recombination in the chloroplast and heterologous recombination in the nucleus. The C. reinhardtii chloroplast genome can be transformed using microprojectile particle bombardment and the nuclear genome has been transformed with both glass bead agitation and electroporation. The biolistic procedures, appears to be the most efficient way of introducing DNA into the chloroplast genome. This is probably because the chloroplast occupies over half of the volume of the cell providing the microprojectile with a large target. Electroporation has been shown to be the most efficient way of introducing DNA into the nuclear genome with maximum transformation frequencies two orders of magnitude higher than obtained using glass beads method.

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